Abstract

The broad topographic framework and erosion surfacemorphology of west central Tasmania predates the earlyPleistocene. The valley systems, however, have beenemphasised by glacial erosion which has played a major rolein shaping the detailed geomorphology of the mountains.Part of an extensive ice cap that developed in the TasmanianCentral Highlands during the late Cainozoic dischargedsouthwards via a major outlet glacier that occupied thevalley of the Derwent River.The heart of the Central Tasmanian ice cap probably lay westof the Du Cane Range. When the ice cover was most extensivethe Derwent Glacier was up to 500 metres thick. It may haveextended to as low as 230 metres above sea level, 70kilometres downstream from its source in the cirques of theDu Cane Range. Two diffluent lobes of this glacier spreadeastwards to merge with other glaciers in the Nive Valley.Other diffluent lobes extended southwards into the upperGordon Valley, and westwards into the upper Franklin andAlma valleys. At the maximum phase the Franklin and Almaglaciers were confluent around Mt. Alma, near the presentjunction of the Collingwood and Franklin rivers.The more westerly glaciers displayed the highest rates ofmass throughput hence glacial landforms are more abundantand better developed in the west. Analysis of the post-depositional modification of theglacial landforms and sediments suggests that at least threeglaciations took place. The first glaciation was probablyearly Pleistocene or late Pliocene in age while the mostrecent and smallest occurred during the late Last GlacialStage.Glaciation would have demanded colder temperatures and anincreased solid precipitation budget, but no major shift inthe direction of snow bearing winds is neccessitated. At nostage was the mean annual air temperature likely to havebeen more than 9° C less than present.The glaciations were probably broadly contemporaneous withthose at similar southern latitudes in Andean Patagonia andSouth Island New Zealand. Like the glaciers of those areasthe ice masses of west central Tasmania were mainly oftemperate maritime character.The glaciations were accompanied by periglacial activitybeyond the limits of the ice. The development of rockglaciers suggests that localised areas of permafrost existedduring the Last Glaciation.The glacial oversteepening has greatly facilitated sloperetreat in areas of high structural anisotropy, particularlyunder periglacial conditions. Interglacial weathering anderosion was comparatively innocuous, although the presenceof a substantial vegetation cover seems to have beencritical to the maintenance of slope stabilty, particularly in steeper and more elevated terrain. The geomorphicevidence does not demand any climate deterioration duringthe Holocene, The most active geomorphological agent of theHolocene interglacial is humankind.